P2PU | A Hands-on Experience with Wireless Sensor Networks (#handsonwsn)

3. Introduction to ZigBee

Zigbee is a specification that is built on top of the IEEE 802.15.4 short range communications standard. Zigbee covers the upper layers of the protocol stack, while 802.15.4 is in charge of MAC and PHY layers.

ZigBee is intended for low-throughput, low-power, low-cost applications. For this reason, it is much simpler than other protocols such as WiFi (IEEE 802.11). It has support for mesh topologies, which means that ZigBee devices relay messages for each other through multiple wireless hops.

The name ZigBee comes from the fact that bees can dance to pass messages to each other, also in a multi-hop fashion.

ZigBee profiles

There are three main players in a ZigBee network:

Coordinator: is the most powerful device. There is a single coordinator in each network. It is the node that creates the network and the other nodes simply join in. Quite often, this is the sink of the WSN, which gathers all the data that is transmitted. One of the coordinator tasks is to assign short addresses.
Router: are intermediate devices. They can relay packets for other nodes. They join a network that already exists and then announce it using beacons. Therefore, they can have “children” nodes that join the network by establishing communication with the router.
End Devices: these are the simplest devices. They cannot forward packets nor have children that depend on them and, quite often, they enter a sleep mode in order to save energy.

ZigBee addressing scheme

ZigBee addressing is hierarchical and ideal for the supported topologies: star, tree and mesh; shown in the image below (figure extracted from (1)).

ZigBee topologies

In ZigBee, each node has two different addresses. The serial number, or hardware address, which is a 64 bits address which is assigned at manufacturing time and is unique in the world.

Then there is the network address, which is only 16 bits-long. It is assigned by the network coordinator and is unique in the network.

There are some special addresses that are possible to highlight:

0x0000 0000 0000 0000 means that the destination is the Coordinator.
0x0000 0000 0000 FFFF is for broadcast.
The short address 0xFFFE is used when the short address is not known or for broadcast.

Each network is identified by a “Personal Area Network Identifier” or PANId. When we configure a network, we must ensure that all the devices are configured with the same PANId.

And also be very careful in assigning different PANIds to different networks. Specially in a classroom situation, if two teams use the same PANId, unexpected results may happen.